1. Field of the Invention
The present invention is addressed to a circuit for the measurement of the impedance of biological and electrochemical systems and more particularly the accurate measurement of the resistance, separate from the total impedance of the systems.
2. Discussion of Background
Biological and electrochemical systems are electrically modeled as a combination of a resistor and a capacitor connected in parallel. The independent determination of either the resistance or the capacitance determines the state of the system. Two examples of this include resistance measured in temperature sensors and capacitance measured in pressure sensors. Therefore it is important to have an impedance sensor that can transduce both phase and magnitude so that independent changes in resistance and capacitance can be separately observed.
It is an inherent characteristic of biological and electrochemical systems that the impedances have significant capacitance, although, in many instances, the characteristic of interest is the resistance. For example, electrodermal measurements or electrochemical "cells" used in monitoring specific reactions and ion concentration require transducing only the resistance. When the capacitive reactance (the capacitive component of impedance) is large compared to the resistive component, accurate measurement of both magnitude and phase of the impedance is required in order to resolve small variations in the resistance. In laboratory environments, lock-in-amplifiers or impedance analyzers are used to make such measurements of voltages across the unknown impedance. These instruments are used in conjunction with voltage dividers to obtain highly sensitive and accurate measurements.
The prior art automated impedance measurment devices use completely analog voltage divider designs which give outputs corresponding to in-phase and quadrature-phase components. Although these devices offer phase sensitivity, the sampling which is required for the analog device is subject to noise and may thus require additional filtering which increases the cost and complexity of such systems.